[14] Aristot. Anal. Post. I. ii. p. 72, a. 25, b. 4. I translate these words in conformity with Themistius, pp. 12-13, and with Mr. Poste’s translation, p. 43. Julius Pacius and M. Barthélemy St. Hilaire render them somewhat differently. They also read ἀμετάπτωτος, while Waitz and Firmin Didot read ἀμετάπειστος, which last seems preferable.
In Aristotle’s time two doctrines had been advanced, in opposition to the preceding theory: (1) Some denied the necessity of any indemonstrable principia, and affirmed the possibility of, demonstrating backwards ad infinitum; (2) Others agreed in denying the necessity of any indemonstrable principia, but contended that demonstration in a circle is valid and legitimate — e.g. that A may be demonstrated by means of B, and B by means of A. Against both these doctrines Aristotle enters his protest. The first of them — the supposition of an interminable regress — he pronounces to be obviously absurd: the second he declares tantamount to proving a thing by itself; the circular demonstration, besides, having been shown to be impossible, except in the First figure, with propositions in which the predicate reciprocates or is co-extensive with the subject — a very small proportion among propositions generally used in demonstrating.[15]
[15] Aristot. Analyt. Post. I. iii. p. 72, b. 5-p. 73, a. 20: ὥστ’ ἐπειδὴ ὀλίγα τοιαῦτα ἐν ταῖς ἀποδείξεσιν, &c.
Demonstrative Science is attained only by syllogizing from necessary premisses, such as cannot possibly be other than they are. The predicate must be (1) de omni, (2) per se, (3) quatenus ipsum, so that it is a Primum Universale; this third characteristic not being realized without the preceding two. First, the predicate must belong, and belong at all times, to everything called by the name of the subject. Next, it must belong thereunto per se, or essentially; that is, either the predicate must be stated in the definition declaring the essence of the subject, or the subject must be stated in the definition declaring the essence of the predicate. The predicate must not be extra-essential to the subject, nor attached to it as an adjunct from without, simply concomitant or accidental. The like distinction holds in regard to events: some are accidentally concomitant sequences which may or may not be realized (e.g., a flash of lightning occurring when a man is on his journey); in others, the conjunction is necessary or causal (as when an animal dies under the sacrificial knife).[16] Both these two characteristics (de omni and per se) are presupposed in the third (quatenus ipsum); but this last implies farther, that the predicate is attached to the subject in the highest universality consistent with truth; i.e., that it is a First Universal, a primary predicate and not a derivative predicate. Thus, the predicate of having its three angles equal to two right angles, is a characteristic not merely de omni and per se, but also a First Universal, applied to a triangle. It is applied to a triangle, quatenus triangle, as a primary predicate. If applied to a subject of higher universality (e.g., to every geometrical figure), it would not be always true. If applied to a subject of lower universality (e.g., to a right-angled triangle or an isosceles triangle), it would be universally true and would be true per se, but it would be a derivative predicate and not a First Universal; it would not be applied to the isosceles quatenus isosceles, for there is a still higher Universal of which it is predicable, being true respecting any triangle you please. Thus, the properties with which Demonstration, or full and absolute Science, is conversant, are de omni, per se, and quatenus ipsum, or Universalia Prima;[17] all of them necessary, such as cannot but be true.
[16] Aristot. Analyt. Post. I. iv. p. 73, a. 21, b. 16.
Τὰ ἄρα λεγόμενα ἐπὶ τῶν ἁπλῶς ἐπιστητῶν καθ’ αὑτὰ οὕτως ὡς ἐνυπάρχειν τοῖς κατηγορουμένοις ἢ ἐνυπάρχεσθαι δι’ αὑτά τέ ἐστι καὶ ἐξ ἀνάγκης (b. 16, seq.). Line must be included in the definition of the opposites straight or curve. Also it is essential to every line that it is either straight or curve. Number must be included in the definition of the opposites odd or even; and to be either odd or even is essentially predicable of every number. You cannot understand what is meant by straight or curve unless you have the notion of a line.
The example given by Aristotle of causal conjunction (the death of an animal under the sacrificial knife) shows that he had in his mind the perfection of Inductive Observation, including full application of the Method of Difference.
[17] Aristot. Analyt. Post. I. iv. p. 73, b. 25-p. 74, a. 3. ὃ τοίνυν τὸ τυχὸν πρῶτον δείκνυται δύο ὀρθὰς ἔχον ἢ ὁτιοῦν ἄλλο, τούτῳ πρώτῳ ὑπάρχει καθόλου, καὶ ἡ ἀπόδειξις καθ’ αὑτὸ τούτου καθόλου ἐστὶ, τῶν δ’ ἄλλων τρόπον τινὰ οὐ καθ’ αὑτό· οὐδὲ τοῦ ἰσοσκέλους οὐκ ἔστι καθόλου ἀλλ’ ἐπὶ πλέον.
About the precise signification of καθόλου in Aristotle, see a valuable note of Bonitz (ad Metaphys. Z. iii.) p. 299; also Waitz (ad Aristot. De Interpr. c. vii.) I. p. 334. Aristotle gives it here, b. 26: καθόλου δὲ λέγω ὃ ἂν κατὰ παντός τε ὑπάρχῃ καὶ καθ’ αὑτὸ καὶ ᾗ αὐτό. Compare Themistius, Paraphr. p. 19, Spengel. Τὸ καθ’ αὑτό is described by Aristotle confusedly. Τὸ καθόλου, is that which is predicable of the subject as a whole or summum genus: τὸ κατὰ παντός, that which is predicable of every individual, either of the summum genus or of any inferior species contained therein. Cf. Analyt. Post. I. xxiv. p. 85, b. 24: ᾧ γὰρ καθ’ αὑτὸ ὑπάρχει τι, τοῦτο αὐτὸ αὑτῷ αἴτιον — the subject is itself the cause or fundamentum of the properties per se. See the explanation and references in Kampe, Die Erkenntniss-theorie des Aristoteles, ch. v. pp. 160-165.
Aristotle remarks that there is great liability to error about these Universalia Prima. We sometimes demonstrate a predicate to be true, universally and per se, of a lower species, without being aware that it might also be demonstrated to be true, universally and per se, of the higher genus to which that species belongs; perhaps, indeed, that higher genus may not yet have obtained a current name. That proportions hold by permutation, was demonstrated severally for numbers, lines, solids, and intervals of time; but this belongs to each of them, not from any separate property of each, but from what is common to all: that, however, which is common to all had received no name, so that it was not known that one demonstration might comprise all the four.[18] In like manner, a man may know that an equilateral and an isosceles triangle have their three angles equal to two right angles, and also that a scalene triangle has its three angles equal to two right angles; yet he may not know (except sophistically and by accident[19]) that a triangle in genere has its three angles equal to two right angles, though there be no other triangles except equilateral, isosceles, and scalene. He does not know that this may be demonstrated of every triangle quatenus triangle. The only way to obtain a certain recognition of Primum Universale, is, to abstract successively from the several conditions of a demonstration respecting the concrete and particular, until the proposition ceases to be true. Thus, you have before you a brazen isosceles triangle, the three angles whereof are equal to two right angles. You may eliminate the condition brazen, and the proposition will still remain true. You may also eliminate the condition isosceles; still the proposition is true. But you cannot eliminate the condition triangle, so as to retain only the higher genus, geometrical figure; for the proposition then ceases to be always true. Triangle is in this case the Primum Universale.[20]